The invention relates to high lumen output fluorescent discharge lamp having high color rendering.
The color rendering ability of a light source is measured with the color rendering index CRI. CRI measures difference between appearance of test colors under artificial light (where the artificial light is emitted by the light source to be measured) and the appearance of the same test colors when seen by light from a blackbody source having the same color temperature as the tested light source. The method to measure the color rendering index is disclosed in “Method of Measuring and Specifying Colour Rendering Properties of Light Sources, 2nd Edition”, International Commission on Illumination, Publication CIE No. 13.2 (TC-3.2) 1974, the contents of which are hereby incorporated by reference. The differences in value, chroma and hue of the light reflected under the light source to be measured and the light source are obtained and summed, the square root of the sum is taken, multiplied by a constant, and subtracted from 100. This calculation is performed for 14 different color standards. The color rendering index for each of these standards is designated Ri, where i=1, . . . , 14. The General Color Rendering Index, Ra, is defined as the average of the first eight indices, R1–R8. The constant has been chosen such that Ra for a standard warm white fluorescent tube is approximately 50. For better illustration, an Ra value of 100 corresponds to a “perfect” light source, i.e. under which a color sample appears exactly as it would appear when illuminated by a “standard” light source, such as an incandescent (black body) lamp or natural daylight, which are perceived as the most “natural” light conditions.
From the above it follows that another factor, the correlated color temperature should be also considered, when assessing the color rendition of a lamp. The correlated color temperature (CCT) value of a light source is defined as the temperature of a black body radiator which would appear to have the same color as the light source in question. The unit of measurement is in Kelvin (K) which determines the warm or cool appearance of a light source. The lower the color temperature, the warmer or more yellow is the appearance. The higher the color temperature, the cooler or bluer is the appearance. Typical color temperatures are 2800K for incandescent, 3000K for halogen, 4200K for cool white, and 5000K for metal halide and daylight fluorescent lamps. Generally, fluorescent lamps with a CCT value of 3200K are used to “imitate” an incandescent light source, while lamps with a CCT value of approx. 5500K are supposed to provide the same or similar illumination as natural daylight.
A similar measure of a lamp is used by photo professionals. This is termed as photographic color temperature, and it takes into consideration the sensitivity curves of various films. The values of the photographic color temperature may be quite different from the CCT value, due to the differences in the method of measurement. Accordingly, it is more appropriate to characterize lamps with their photographic color temperature, instead of the CCT, if these are destined for use in photography or cinematography. Photographic color temperature is measured by specialized color meters, where e.g. the meters marketed by Minolta Corp, Japan are considered as virtual standards.
There are certain applications where good color rendition is very important. Such applications are illumination in commercial units, where the true color perception of products are desired, such as clothing stores, fresh food stores. Another important application is photo and cinema studios, which normally need very intensive illumination. Further, traditional light sources in cinema studios generate so much heat that extra cooling of the rooms may be needed. Therefore, these latter applications need light sources that have not only good color rendition, but are also energy efficient.
Various attempts were made to make fluorescent light sources with improved color rendering properties. It is normally sought to improve or modify the color rendering properties by blending different types of phosphors. For example, U.S. Pat. No. 5,028,839 and U.S. Pat. No. 5,539,276 disclose fluorescent lamps, primarily for use in aquaria, which have a fluorescent layer composed of various phosphors, having different emission peaks and half-width values.
U.S. Pat. No. 6,525,460 discloses fluorescent lamp in the form of a light tube having very high color rendition properties. This known lamp comprises a phosphor-containing layer made of a blend of various phosphors. The lamp has an Ra value greater than 96 and a CCT between 2700K and 6600K. Specifically, the lamp provides very high R values for the colors Saturated Red, Saturated Yellow, Flesh Tone and Vegetable Green. However, in order to obtain these parameters, the phosphor layer of the lamp disclosed in U.S. Pat. No. 6,525,460 also comprises a filter, which is effective in the 400–450 nm range. This filter has a negative effect on the efficiency of the lamp, and also adds difficulty to the manufacturing process of the lamp, because the proportion of an additional component must be controlled.
Recently, there is a trend towards smaller form factors in the fluorescent lamp market, and it is desired to achieve the same or better lighting performance with compact fluorescent lamps which could be traditionally achieved only with relatively large light tubes. However, if the same light output is to be produced in a smaller discharge vessel, it will inevitably increase the wall load, i.e. the amount of energy falling on a unit area of the phosphor. For example, if a traditional light tube with a diameter of 38 mm is to be replaced by a compact fluorescent lamp with a diameter of approx. 15 mm, such as a lamp for a 2G11 socket, the wall load will be approx. fourfold. This means that the phosphor will be subjected to a much higher load, and certain components of the phosphor will tend to deteriorate, due to oxidization or other processes. As a result, a general degradation of the luminous parameters of the lamp will be observed. For example, the phosphor (Sr,Mg,Ca)3(PO4)2:Sn2+ used in the phosphor blend of the lamp disclosed in U.S. Pat. No. 6,525,460 exhibits a marked degradation tendency with increasing lumen output. These effects will be even more significant at the bends of the discharge tube.
Therefore, it is an imperative to use phosphor blends that have high conversion efficiency, so that a high lumen output can be achieved with relatively low power consumption, thereby also reducing the load on the phosphor. Since the conversion efficiency of a phosphor is difficult to measure, it is usual to measure the luminous efficiency of fluorescent lamps (also termed as efficacy). Efficacy is the industry term for the amount of light produced per watt of electricity, and therefore it is quite comparable to efficiency. Efficacy is the rate at which a light bulb is able to convert electrical power (watts) into light (lumens), expressed in terms of lumens per watt (LPW). The efficacy of the lamp depends on a number of factors beside the conversion efficiency of the phosphor, but for similar discharge configurations and similar discharge volume geometries, the differences in the efficacy will be primarily determined by the conversion efficiency of the phosphor.
Therefore, there is a need for a fluorescent lamp having a stable phosphor composition, which provides at the same time outstanding color rendering, preferably at different color temperatures, combined with high luminous efficiency. Also, there is a need for a fluorescent lamp which contains only a few phosphors in its phosphor composition, and thus may be produced with relative ease.